A vacuum sewage collection system is a system in which sewage water is collected by causing a vacuum in a sewer (referred to not as a complete vacuum but as a decompressed state) and by utilizing the pressure difference from atmospheric pressure.
FIG. 3 shows an example of the arrangement of this vacuum sewerage system.
Sewage water discharged from a home or factory sanitary facilities flows into a vacuum valve unit (relay unit) 32 through an inflow pipe 31. The sewage water is then led from this vacuum valve unit 32 to a vacuum station 34 through a vacuum sewer 33 and is thereafter led to a sewage treatment system through a pressure feed pump 35 and a pressure feed pipe 36.
In this vacuum station 34, sewage water in a receiving tank 38 is fed to an ejector 39 by sewage circulation pump 37. The vacuum sewer 33 is thereby evacuated so that sewage water is collected in the vacuum station 34.
The vacuum valve unit 32 serves for relaying between the sewage source and the vacuum station 34, and has a tank 40 into which sewage water from the inlet pipe 31 flows, a suction pipe 41 for drawing sewage water in the tank 40 and supplying the drawn sewage water to the vacuum sewer 33, a vacuum valve 42 provided in the suction pipe 41, a controller 43 for operating the vacuum valve 42, and so on. For the vacuum valve 42, a negative pressure in the vacuum sewer 33 is used as a driving power source. In the illustration, an air pipe is indicated at 44, an inspection hole is indicated at 45, an air pipe is indicated at 46, and lifts are indicated at 50. Ordinarily, a plurality of vacuum valve units are connected to a vacuum sewer.
Such a vacuum sewage collection system does not require, in laying a pipe line, a continuous gradient such as that in a natural downflow type sewerage and has the following advantages.
1 Since the pipe line laying depth is small, the sewer construction cost can be reduced remarkably.
2 It enables sewerage construction in an area where laying of sewers is difficult because of a high underground water level or difficulty in excavation due to the existence of a base rock or for other reasons.
3 Construction under a winding lane or the like is easy.
4 Because of forced intermittent high-speed collection of a gas-liquid mixture using a vacuum, the system is free of clogging in pipe lines and piping using small-diameter pipes is possible.
In a vacuum sewage collection system, the transportable range (sewage collection basin) is a range in which the degree of vacuum at ends of vacuum sewers is maintained at a negative pressure of 1,000 to 2,500 mmAq. Accordingly, in the case of a system having no factor of reducing the degree of vacuum in vacuum sewage pipe lines, the transportable range can be obtained as a value proportional to the value which is obtained by subtracting the necessary negative pressure of 1,000 to 2,500 mmAq at the end from the degree of vacuum H.sub.0 generated in the vacuum station.
If there is a rising gradient in a vacuum sewage pipe line in such a vacuum sewage collection system, the head at the rising gradient consumes the vacuum generated in the vacuum station to cause a reduction in the degree of vacuum, resulting in a reduction in the transportable range. For example, if, in a ground configuration where there is an obstacle (e.g., a river), a vacuum sewer 33 is embedded so as to pass under or over the obstacle, i.e., a river or the like, as shown in FIG. 4 or 5, the head between A and B is H.sub.1 or H.sub.2. By this head H.sub.1 or H.sub.2, the degree of vacuum H.sub.0 of the vacuum station is correspondingly reduced (H.sub.0 -(H.sub.1 or H.sub.2)). The transportable range in this case is proportional to a value obtained by subtracting the above-mentioned necessary negative pressure 1,000 to 2,500 mmAq at the end from (H.sub.0 -(H.sub.1 or H.sub.2)). Thus, the transportable range in this case is much smaller than the transportable range in the case of a flat ground configuration.
For this reason, the development of a technique is expected which enables, in a case where an obstacle is formed in a vacuum sewage pipe line between a sewage generation source and a vacuum station, prevention of a reduction in the degree of vacuum due to a head of the obstacle to extend the sewage transportable range.